The present invention relates to the production of diaryl carbonates. More particularly, it relates to the continuous production of diaryl carbonates in the molten state using single and double recirculation loops.
The prior art processes for the production of diaryl carbonates include processes based on the use of armomatic hydroxy compounds reacted with phosgene in the presence of quaternary ammonium compounds or other tertiary amines as catalysts in the presence of water-immiscible solvents. It is also known to introduce phosgene into a highly alkaline solution of o-cresol at elevated temperatures to produce diarylcarbonates. See German Pat. Publication, OL No. 2509036, incorporated herein by reference.
Commercially, diaryl carbonates are made by a batch slurry process. Such a process is used to make diphenyl carbonates by reacting phosgene with phenol in the presence of caustic at a pH of 10-11, at 55.degree.-60.degree. C. The product is recovered as solid particles which are generally melted for hot water washing to remove the impurities. The major disadvantages of the process are the large volumes of water needed and solids handled and the poor quality of product due to the entrainment of phenyl chloroformate, salt and other impurities. The prior art batch processes have produced diaryl carbonates having phenylchlorocarbonic esters as byproduts and have yielded less than optimum yields of the desired product.
A leading use for diaryl carbonates is as reactants for making polycarbonates in transesterification reactions. If a diaryl carbonate such as diphenyl carbonate is contaminated with phenyl chloroformate, it will be difficult or impossible to produce a commercial grade of a polycarbonate molding resin.
Therefore, it is a principal object of this invention to provide a process for producing a diarylcarbonate in a highly pure form with high yields.
It is also an object of this invention to provide a process for making diaryl carbonates in high yields without requiring a catalyst.
It is also an object of this invention to provide a process for making diaryl carbonates which is operated continuously to produce diaryl carbonates in a highly pure form in high yields.